Five journalists including the chief executive of the Unity Weekly News were arrested on Friday and Saturday, according to colleagues, who said they are facing charges over a report in January.
"Family members were informed last night by the authorities that they could visit them in Insein prison and told to hire lawyers," said Unity editor Aung Thura Ko Ko.
"They have been charged under the official secrets act," he added.
The newspaper said the arrests were linked to an article claiming that the country's military was operating a chemical weapons factory in Pauk, in the central Magway region, under the instructions of former strongman junta chief Than Shwe.
Myanmar's military ruled the country with an iron fist for nearly five decades and locked it in isolation, banning dissent and imprisoning critics and journalists.
A quasi-civilian government took power in 2011 and ushered in much-heralded reforms including easing media restrictions.
Detained reporters were freed, draconian pre-publication censorship was ended and private newspapers were allowed to publish daily.
The army, which retains a powerful hold over the government and parliament, continues to be accused of rights abuses, particularly in ethnic minority areas.
But the United Nations has welcomed a reduction in the use of child soldiers.
Neither local police nor the government spokesman were available to comment on the report or subsequent arrests.
Unity Weekly quoted testimony from local people and workers and included pictures of the buildings.
The New York-based Committee to Protect Journalists (CPJ) raised alarm about the developments.
"The fact that journalists can be charged with revealing state secrets shows how desperately Burma needs meaningful legal reform," said Shawn Crispin, CPJ's senior Southeast Asia representative, using the country's former name.
"Weapons proliferation issues are central to Burma's political narrative and journalists should not be threatened or arrested for reporting on topics of national and international importance."
In January last year Myanmar denied accusations it had used chemical weapons against ethnic minority rebels in the northern state of Kachin.
"Our military never uses chemical weapons and we have no intention to use them at all. I think the KIA (Kachin Independence Army) is accusing us wrongly," presidential spokesman Ye Htut said at the time.
The US Treasury in December levelled sanctions against a Myanmar military official and three businesses in the country for trading arms with North Korea.
The Treasury said the sanctions, which forbid any US person or entity from doing business with those blacklisted, "does not generally target" the government of Myanmar.